1. Field of the Invention
The present invention relates to a coating composition which can be used in semiconductor preparation, especially in crystalline silicon solar cell preparation. The coating composition can be coated onto a substrate to form a passivation layer thereon, so that the recombination possibility of electron-hole pairs can be reduced and the photovoltaic conversion efficiency can be improved.
2. Descriptions of the Related Art
With the rapid development of technology and economy, the demand for energy has grown significantly. The energy storages used conventionally, including petroleum, natural gas and coal, have gradually decreased. Solar energy is now one of the most valued and important new energy sources because it has low pollution and is easy to obtain.
FIG. 1 shows a schematic diagram of a typical solar cell, wherein an n type doping layer 2 is formed on a p type silicon semiconductor substrate 1 by doping. An anti-reflective layer 3 (such as silicon nitride) and electrodes 4 are subsequently formed on the n type doping layer 2. A back electrode 5 is formed on the other side of the p type silicon semiconductor substrate 1. The conducting types of the silicon semiconductor substrate 1 and the doping layer 2 may be exchanged with each other (i.e., a combination of an n type silicon semiconductor substrate and a p type doping layer is possible). When light irradiates the internal electric field formed between the p type silicon semiconductor substrate 1 and the n type doping layer 2, the electrons of the silicon atoms will be excited to generate light-generated electron-hole pairs. The carriers of the electron-hole pairs will then concentrate on both sides of the cell due to the electric field; and the electric power inside the cell can be utilized by means of connecting an external circuit to the electrodes.
However, the electron-hole pairs are usually recombined inside the solar cell and cannot be utilized, resulting in the reduced photovoltaic conversion efficiency of the solar cell. It is known that such recombination can be reduced through a “back surface field effect.” In detail, an electro-negative passivation layer (such as an aluminum oxide layer) is formed between the back electrode 5 and the p type silicon semiconductor substrate 1 to restrict the migrating direction of the minority carriers. By such means, the migration of electrons toward the back electrode 5 can be reduced, thereby, the recombination possibility of electron-hole pairs can be reduced (i.e. lengthening the life-time of the minority carriers) and the photovoltaic conversion efficiency can be thus, increased. Said aluminum oxide layer may be provided by a dry process, such as atomic layer deposition (ALD), chemical vapor deposition (CVD), sputtering, etc., or a wet process. In the dry process, a high vacuum environment is required and the growth rate of aluminum oxide is extremely slow, resulting in high costs. In contrast, the wet process can be performed by the sol-gel method which is relatively simple and less costly. However, the particle size of the aluminum oxide layer prepared by the sol-gel method is uncontrollable and non-uniform and the stability of the solution of the sol-gel process is poor, therefore, it is hard to control and commercialize the process.